Latitudinal oscillations of plasma within the Io torus

Abstract

We calculate the equilibrium latitude and the period of oscillations about this equilibrium latitude for plasma in a centrifugally dominated tilted dipole magnetic field representing Jupiter's inner magnetosphere. For a hot plasma the equilibrium latitude is the magnetic equator; for a cold plasma it is the centrifugal equator; for a warm plasma it is somewhere in between. In the small-tilt-angle approximation, the latitudinal oscillation period of a cold particle in the centrifugal potential field is Po = Pj/31/2, where Pj is the rotation period of Jupiter. For a warm plasma the magnetic mirror force causes the bounce period to be decreased slightly. These results are applicable to the plasma injected by Io to form the torus of plasma surrounding Jupiter. We adopt an illustrative model in which atoms are sputtered from the Jupiter-facing hemisphere of Io and escape Io's gravity to be subsequently ionized some distance from Io. Ionization generally does not take place at the equilibrium latitude, and the resulting latitudinal oscillations provide an explanation for the irregularities in electron concentration within the torus, as reported by the radio astronomy experiment aboard Voyager 1.

Sodium in Io's Extended Atmosphere

“…and bounce velocities can be substantial. It is well-established (Cummings et al, 1980) that ions formed at maximum magnetic latitudes (±7°) will fall toward the plasma equator due to centrifugal and magnetic forces. Such ions have speeds of .....,12 km sec-1 at the plasma equator, but vanishingly small velocities when they return to Io's position at higher magnetic latitude.…”

“…The plasma torus is known to be nonuniform (Morgan, 1985;Cummings et al, 1980), and it is possible that random clumps of plasma ten times denser than normal can bounce coherently along field lines for extended periods. We originally proposed that the jets were generated by 200 the interaction of such clumps with the extended neutral clouds (Schneider et al, 1984), but this explanation no longer appears likely.…”

Dynamic features of Io's extended sodium distributions

“…Cummings et al (1980) showed that the field-aligned motion of picked-up ions could be approximated as a simple harmonic oscillation and that the period of oscillation could be described as a function of v gyro . Schneider et al (1991) estimated that the period of field-aligned oscillation was ∼5.6 h, which corresponds to a gyrovelocity of ∼13 km/s.…”

“…In the molecular ion destruction process, newborn picked-up ions swept downstream of Io begin a coherent bouncing motion along Jupiter's magnetic field line (Cummings et al, 1980). The velocity of the ions is described as…”

“…Schneider et al (1991) and Wilson and Schneider (1994) observed a D-line emission distribution encircling Jupiter that exhibited a time-variable structure. They argued that the encircling structure is formed along a streamline of newborn ions from Io, based on the theory proposed by Cummings et al (1980). They considered the dissociation and dissociative recombination processes of sodium-bearing molecular ions to be the source of the streamline.…”